Possible Earth-like planet could hold water; scientists cautious

L. Calcada / European Southern Observatory

This artist's representation shows a planet, right, orbiting the star Alpha Centauri B, center, a member of the triple star system that is closest to Earth. This exoplanet sat too close to its star for liquid water to exist. Now, another team of scientists says it has discovered a super-Earth planet some 44 million light-years away in its system's "habitable zone."

This artist's representation shows a planet, right, orbiting the star Alpha Centauri B, center, a member of the triple star system that is closest to Earth. This exoplanet sat too close to its star for liquid water to exist. Now, another team of scientists says it has discovered a super-Earth planet some 44 million light-years away in its system's "habitable zone." (L. Calcada / European Southern Observatory)

By taking a fresh look at old data, an international team of astronomers has discovered a possible new super-Earth planet relatively nearby that could potentially hold liquid water, scientists said Wednesday.

The research, released by the journal Astonomy & Astrophysics, used a novel technique to analyze existing measurements of a nearby star. The paper drew some praise even as other experts in the field eyed the results with caution.

The finding adds three planet candidates around the dwarf star HD 40307 some 44 light-years away, for a total of six. (Three planets were discovered around HD 40307 in 2008.) Five of the planets are clustered close in to the star, nearer than Mercury’s orbit around the Sun.

One of the three new finds – a planet with seven times the mass of Earth – lies far enough away to be in what’s known as the habitable zone, where a planet could support liquid water and perhaps life.

“It’s likely that it’s sufficient in mass that it does have an atmosphere. But we don’t know. It depends,” study co-author Hugh Jones, a University of Hertfordshire astronomer, said in an interview.

It’s difficult to draw extensive conclusions about such super-Earths – whether they’re gassy like Jupiter or rocky like Mars, whether they’d have a thick or thin atmosphere – because no such mid-range planet exists in our own solar system.

But the planet, HD 40307-g, is far enough way from its star to be able to rotate freely and possibly have a night and day, Jones said, making it even more likely to have an Earth-like climate. (The closer-in planets are gravitationally locked into the star’s motion, just as the moon is locked into Earth's, which is why we only ever see one side of it.)

Though the planet appears somewhat closer to its star than Earth is to the sun, the dwarf star is also smaller and dimmer. The whole system is far closer to Earth than the star Kepler 22, whose Earth-like planet sits about 600 light-years away.

The scientists found the new planet by looking through old measurements collected by the 3.6-meter telescope at La Silla Observatory in Chile. They used a technique called radial velocity spectroscopy, which takes advantage of the Doppler effect.

A planet’s gravity tugs on its star, causing the star to wobble a bit. When the star moves slightly toward us, it squeezes the light, making it bluer; and when the star moves away, the wavelengths of light stretch out, making it redder. The scientists measure that distortion in the light and figure out if a planet is causing it. The wider the wobble, the more massive the planet.

But scientists need to separate that signal from a lot of “noise” caused by the star’s own activity, as dark spots grow and shrink on its surface, affecting the light it emits. At such great distances, making that distinction is an exceedingly difficult task.

But the scientists had noticed a key fact: Stellar activity seems to register more in the blue half of the starlight, making it a bit of a mess to read. The red half of a star’s light fingerprint, however, goes relatively undisturbed by the star's ups and downs. When the team looked only at the red side of the light, the planetary signals suddenly emerged.

“The interesting thing is that if they threw out some of their data, they got more precise measurements,” Eric Ford, an astronomer at the University of Florida at Gainesville, said in an interview.

This new technique, he added, could influence future instrument design. “So what we thought were the limitations of stars being variable may be something that we can work around, by restricting our attention to some of the redder wavelengths.”

The finding has yet to be confirmed by other analysis or observations. Not everyone is convinced this particular planet exists.

Francesco Pepe, one of the Geneva Observatory scientists who announced the discovery of a nearby Earth-sized planet just last month – using the same telescope at La Silla – said his team had already seen the signals referred to by this new paper. In fact, they had originally produced the data in question.

“These signals are, however, at the edge of detectability and some doubts remain(ed) on their planetary nature,” Pepe wrote in an email (parentheses his). “For this reason we have been following up this star continuously. It is our policy to exclude any possible other explanation and to collect sufficient data to confirm the (possible) additional planets.”

Other researchers expressed far greater doubt.

"The exquisitely high-quality data required to find an Earth-like planet doesn't just happen by chance," UC Berkeley astronomer Geoff Marcy wrote in an email.

Scientists have become increasingly antsy about false positives. Ford recalled two “Earth-like” planets announced in 2010 to be circling Gliese 581 whose existence was almost immediately thrown into question, launching a flurry of quarreling papers in the astronomical community. Their existence is still somewhat in dispute.

The leaders of that research, Steven Vogt of UC Santa Cruz and Paul Butler of the Carnegie Institute of Washington, are two of the coauthors of the new paper. And, to come full circle, Pepe was one of the scientists to contest the earlier finding, again using La Silla data as a source.

“There’s the ideal science and there’s the science that involves humans,” Ford said.

In an ideal world, he said, who collects and analyzes the data wouldn't matter. But after such protracted battles, “people probably don’t forget things like that.”

Regardless, he added, the findings are promising, and part of the planet-hunting learning curve.

“While this is a nice advance and I suspect all these planets will probably pan out to be real,” Ford said, “it’s certainly just one of many small steps toward finding Earth-mass planets in the habitable zone.”